Hydrophone calibrator

ABSTRACT

A compact, complete, portable hydrophone calibrator or tester suitable for evaluating or calibrating a hydrophone and particularly for checking the sensitivity of a hydrophone. In the embodiment described below, the hydrophone is placed in a waterfilled chamber which is surrounded by a first piezo-ceramic cylinder which acts as a sinusoidal pressure source and a second piezo-ceramic cylinder which, together with the hydrophone, receives the sound produced by the first cylinder. The output of the second cylinder and the hydrophone are compared to determine the sensitivity of the hydrophone, after the second cylinder is calibrated by the introduction of a step pressure change to set the absolute calibration level in dB relative to 1 volt per microbar.

United v States Patent Trott 51 Apr. 25, 1972 [54% HHYDROPHONECALIBRATORPrimary Examiner-Richard A. Farley Attorney-Cushman, Darby & Cushman[72111 lnventor: Winfield James Trott, 3907 Meno Drive, Doraville,[2211: Filed: Sept 1969 A compact, complete, portable hydrophonecalibrator or [211 A L N 361,036 tester suitable for evaluating orcalibrating a hydrophone and particularly for checking the sensitivityof a hydrophone. ln the embodiment described below, the hydrophone isplaced in [52] L8. Cl. ..340/5 C, 73/1 DV, 181/3043 a watepfilled h b hih i u rounded by a first piezot H t 7 04b 14/00 H ceramictcylindelttwhich acts as a sinusoidal pressure source 3 f 1/0 5 AP and asecond piezo-ceramic cylinder which, together with the 1 0 care hhydrophone, receives the sound produced by the first cylinder. Theoutput of the second cylinder and the hydrophone are compared todetermine the sensitivity of the [56] References cued hydrophone, afterthe second cylinder is calibrated by the in- UNITED STATES PATENTStroduction of a step pressure change to set the absolute 3 121 2112/1964 E k I 340/8 calibration level in dB relative to 1 volt permicrobar.

, s in et a OTHER PUBLIC O S 18 Claims, 2 Drawing Figures McMahon. Jour.Acous. Soc. Amer., Dec. 1964, pp. 23ll AVFUI 75/76 406- flMM/F/E/e17:75am? F/Arfe HMI /F/EE 7? f/VOWPI/O/VE WM 1 M/JEZ //VP//7' P544:[/1556 4& revwsm/rn/m fl/"pl/F/f'e Ava/"7'56 ('fl/lVIf/Z 41/ 4 wait-'55/76 1 177575? Jen/5e 0501447442 1 PATENTED APR 2 5 :912

SHEET 2 BF 2 mvmon Win i762 Jwislzarr UP NQQQQQ ATTORNEYS HYDROPHONECALIBRATOR BRIEF DESCRIPTION OF THE PRIOR ART AND SUMMARY OF THEINVENTION waves, the sciencexof acoustics has become increasinglyimportant in transmitting underwater information and in extending ourknowledge of the seas. Acousticalmethods have been i found useful inmany diverse applications from locating submarines, to determining thephysical structure of the ocean bottom and organisms found thereon.

Transducer is a term .widely used to designate a broad category ofdevices which are used for the generation and/or receptionof underwatersound. The term projector is com- .monly applied to a generator ofsound, while hydrophone refersto a receiver. Generally, a projectorconverts electrical energy to .motional, mechanical energy which isimparted as acoustical waves to a body of water, while a hydrophoneconverts such waves when, received as mechanical energy, into electricalenergy in wave forms which can then be recorded and analyzed. Manytransducers are capable of operating both asa hydrophone and as aprojector.

To. insure that dataobtained with hydrophones is accurate, it is.necessary to continually re-evaluate and test the hydrophones to makesure that the performance of each is within designated limits and toremove those hydrophones which are performing .unsatisfactorily. Suchchecks are preferably made at frequent intervals and accordingly thereis great need for a simple, accurate, economical device which can beeasily mounted aboard a ship and which can quickly and simply establishor verify the calibration of a hydrophone,

preferably in the frequency range where degradation is most apttoappear.

Techniques for checking hydrophones currently in use have all proved toocomplex, too .uneconomical or too inaccurate. For example, thetwo-projector-null method, while relatively accurate, requires far toocomplex circuitry and measurements to be a practical procedure fornormal checking. The

, vibrating columnof. water method is limited in its frequencyrangewReciprocity calibration in; a free field is also limited at thelowfrequency by the depth of the water and is likewise a complexmeasurement.

The present invention relates to a simple method and apparatus forevaluating or calibrating a hydrophone wherein thehydrophone is simplyinserted into a water-filled calibrating ascharnber located within twopiezoceramic cylinders mounted along substantially the same axis. One ofthe cylinders-acts as a sinusoidal pressure source and is preferably.driven through the frequency range of interest by a variable frequency.oscillator. The .other receiving piezoceramic BRIEF DESCRIPTION OF THEDRAWINGS FIG... .1 shows a cutaway view of the novel hydrophonecalibrator of this invention.

FIG; 2 shows a block diagram of the hydrophone calibrator and theelectrical elements for producing an indication of the sensitivity .ofthe hydrophone tested.

DETAILED DESCRIPTION OF THE DRAWINGS Reference. is now made to FIG. 1which shows a cut-away viewof the novelhydrophone calibrator or testerof this invention. As mentioned briefly above, this calibrator isintended to be a complete, compact, portable unit, which is preferablypackaged in a suitcase style carrying case so as to be easily manageableand to be convenient to wherever hydrophones need testing. The devicecan be bench-mounted for shipboard use or can be otherwise mountedhowever desirable.

Further, the unique hydrophone tester shown in FIGS. 1 and 2 has beenparticularly designed for the calibration of hydrophones in a frequencyrange of 1 Hz to 5 KHz, which is the range in which degradation is mostlikely to appear, although testing outside this preferred range is ofcourse possible. This device is an absolute calibrating device formeasuring the sensitivity of a hydrophone in volts per microbar, whichis the free field voltage sensitivity, so long as the hydrophone volumecompliance is no greater than the volume compliance of the waterdisplaced by the hydrophone. Most hydrophones at the low frequency rangefor which this device is particularly designed satisfy this compliancecondition.

The arrangement shown in FIGS. 1 and 2 is a complete system consistingof a source of sinusoidal pressure of known magnitude in a closedchamber, an electronic drive for the source and receiving circuitry. Thesource of pressure in the embodiment shown in FIGS. 1 and 2 is simply apiezo-ceramic cylinder 22 which is mounted about the area into which thehydrophone is inserted for testing. Cylinder 22 is mounted between tworubber ring seals 24 and 26 which serve to hold it firmly in place. Seal26 is attached to a bottom plate 28 while seal 24 connects to a divisionring 30 between the transmitting cylinder 22 and the measuring cylinder32 which is a likewise piezo-ceramic cylinder preferably identical tocylinder 22. An exterior supporting tube 31 surrounds both cylinders 22and 32 and also provides an air filled chamber 33 around the twocylinders 22 and 32. Rubber ring seals 35 and 37 hold cylinder 32 inplace and ring seal 37 connects to upper plate 39 as shown in FIG. 1.

At low frequencies, the transmitting piezo-ceramic cylinder 22 isessential a capacitor driving a mechanical compliance to which thepiezo-ceramic receiving cylinder 32 is coupled. Receiving cylinder 32 isalso substantially a capacitance at these frequencies. A constantvoltage drive of, for example, 10 volts thus produces an open circuitoutput voltage at receiving cylinder 32 of, for example, -18 dB relativeto 1 volt per microbar at frequencies up to about 500 Hz.

Above this frequency, the chamber impedence is no longer a truecompliance due to the approaching longitudinal resonance. Thus, thel0-volt drive in effect produces a sound pressure in the chamber thatincreases with frequency. At the chamber resonance, the pressure isabout 17 dB above the level at frequencies below 500 Hz. The receivingcylinder 32, however, continues to measure the sinusoidal pressure inthe region of the hydrophones active elements, and for this reason, theratio of the hydrophone output voltage to the output voltage of thereceiving cylinder 32 as shown on a D.C. meter or as recorded on a stripchart recorder is used as a measure of sensitivity rather than theabsolute output level of the hydrophone being tested.

A typical conventional hydrophone is calibrated as follows. First, thehydrophone being tested is inserted into the hollow test chamber 44which is filled with water, castor oil or similar liquid. Alever-operated clamping ring preferably seals the chamber for up to1,000 psi and a pinion gear 49, which is illustrated in FIG. 1, isincluded to accomplish that sealing. Then, piezo-ceramic cylinder 22 isdriven by a conventional oscillator 40, as shown in FIG. 2, through aconventional driver circuit 42. Oscillator 40 may be simply a variablefrequency, three decade oscillator capable of producing at least 10volts rms.

The operator next introduces a precision pressure step change of, forexample, 10,000 microbars to set the absolute calibration level in dBrelative to 1 volt per microbar. This is accomplished by switching themanometer tube connected to the interior of the hydrophone chamber 44.For example, when the conventional switch 50 is operated to disconnectmanometer 54 and to connect manometer 52, which is roughly centimetershigher than the manometer 54 to the line 56, which connects to thechamber 44, a 10,000 microbar step pressure change in chamber 44 takesplace. The output of oscillator 40 can then be adjusted to generate asinusoidal pressure and after this is accomplished the sensitivity ofthe hydrophone being tested versus frequency can be easily obtained byvarying the frequency of the oscillator 40 over the desired frequencyrange.

As mentioned briefly above, the receiving peizo-ceramic cylinder 32 ismounted axially with the transmitting cylinder 22 and is preferablycalibrated by a hydrostatic pressure step change such as equivalent to10 centimeters of water height. The sensitivity of this piezo-ceramiccylinder 32 for one embodiment was found to be approximately 88 dBrelative to l volt/microbar over the frequency range of the calibrator.Due to the change in chamber acoustical impedance as the frequencyapproaches the half-wave resonance of the chamber 44 the output voltagesof the hydrophone being tested and the receiving ceramic cylinder 32increase about 17 dB at frequencies above 500 KHz. For this reason, theratio of the two output voltages is used to produce a sensitivityindication which remains constant when the sensitivity of the hydrophonein fact remains constant.

The electrical output signal from the hydrophone being tested is firstrouted to an input amplifier 56 and from there to a peak detector 58.The output of the peak detector 58 is passed through a filter 60 to alogarithmic amplifier 62 which produces a signal which is in effect thelogarith to some convenient base of the output of the filter 60. Theoutput of the receiving cylinder 32 is similarly passed through an inputamplifier 64, a peak detector 66, a filter 68 and a logarithmicamplifier 70. The outputs of the amplifiers 62 and 70 are applied to asum-difference amplifier 72 and the output of the amplifier 72 isdisplayed on a conventional D.C. meter 74. The output of the meter canalso be applied to any other suitable circuitry and, of course,represents the analog D.C. value of the output of amplifier 72. Thus,the output of amplifier 72 is in effect a function of the ratio of theoutput voltages of the receiving cylinder 32 and the hydrophone beingtested. From this ratio function, the sensitivity of the hydrophone canbe checked over the frequency range of interest, which, as mentioned,briefiy above, is normally 1 Hz to 5 KHz.

The two ceramic cylinders 22 and 32 can be potted in polyurthane orsilicone resin, if desired. Further the cylinders can be shielded fromthe conductivity of the water within the cavity 44 in which thehydrophone is placed by means of a rubber boot, butyl or neoprene. InFIG. 1, a polyurethane liner 80 separates the chamber 44 from thecylinders 22 and 32. Means can also be provided for varying thetemperature and hydrostatic pressure within chamber 44 or otherwisecalibratin g or testing hydrophones at other than ambient conditions.

As mentioned above, there are a number of defects which can occur in thehydrophone which substantially affect the accuracy of data obtained withit. For example, a piezo-electric hydrophone with pre-amplifier outputwill normally have a constant free field voltage sensitivity over abroad frequency range below the first resonance. At very lowfrequencies, the capacitive reaction to the piezo-electric crystal orceramic element will normally equal the electrical leakage resistanceacross it and the sensitivity will be roughly 3 dB relative to theconstant sensitivity at higher frequencies. Sensitivity at lowerfrequencies will continue to drop 6 dB per octave. If moisture haspenetrated to the piezo-electric element, however, then the lowfrequency cut-off will substantially increase and this form ofdegradation can be easily detected by the novel calibrator of thisinvention.

Further, some piezo-electrical elements, such as those made withAmmonium Dihydrogen Phosphate, require that the surface normal to two ofthe coordinate axes be isolated from the sound field, and this issometimes accomplished by cementing a pressure release material on thesesurfaces. If the hydrophone element is enclosed in an oil-filled boot,then the oil may eventually saturate the pressure release material,reducing the isolation and lowering the sensitivity. This failure willalso be detected by the calibrator of this invention.

In an oil coupled construction, the oil is carefully deareated duringconstruction. The presence of small air bubbles in the oil due to poorconstruction or subsequent leakage of air into the oil will producespurious resonances. For example, a bubble resonating at 2,500 Hz (0.1inch diameter) will affect the hydrophone sensitivity 2 or 3 dB atresonance and will lower the output voltage of the calibrator by about 1dB below resonance. Smaller bubbles, which resonate at higherfrequencies will produce a lesser effect. Failure due to poor cementbonds that reduce the accoustic coupling or fractured piezoelectricelements that reduce the element capacitance will lower the sensitivitywithin the range of the calibrator and be thus detected.

Sensitivity above the range of the calibrator will generally remainconstant except for diffraction effects and resonances of the elementand hydrophone structure, both a function of hydrophone dimensions.Degradation in this upper frequency range can often be detected asdegradation within the frequency range of the calibrator, a far simplermethod than measurement in a free field.

Thus this novel hydrophone described above is portable, compact,complete and capable of determining sensitivity over a wide frequencyrange, for example, 1 Hz to 5 KHz is also capable of quickly and simplydetecting defective hydrophones or deteriorated performance andeliminates the necessity for returning such hydrophones from the fieldto the lab for calibration. As mentioned briefly, an external recordercan also be associated with the equipment to provide a hard copy ofcalibration data. Further, ambient pressure in the chamber can bevaried, if desired, from 0 to 1,000 psi or higher by attaching anitrogen or air bottle with regulator to a standard fitting on thecalibrator.

Many changes and modifications from the novel hydrophone of thisinvention are indeed possible without departing from the spirit of theinvention. Accordingly, the scope of the invention is intended to belimited only by the scope of the appended claims.

What is claimed is:

1. A hydrophone testing apparatus comprising:

chamber means having a cylindrically shaped interior cavity forreceiving the hydrophone to be tested, and adapted to be filled with aliquid, and having an inner cylindrical wall bounding said cavity afirst cylindrical tube of piezo-ceramic material forming part of saidinner wall and extending about said cavity for imparting sound to saidliquid,

a second cylindrical tube of piezo-ceramic material forming part of saidinner wall and extending about said cavity axially aligned with saidfirst tube for receiving the sound imparted to said liquid by said firsttube, and producing a first electrical signal,

means for producing a second electrical signal from the sound receivedby said hydrophone being tested and means for comparing said first andsecond electrical signals to test said hydrophone.

2. Apparatus as in claim 1 ends of said tubes.

3. An apparatus as in claim 1 wherein said liquid is water.

4. An apparatus as in claim 1 including means for producing a stepchange of pressure in said chamber means to calibrate said second tube.v

5. An apparatus as in claim 4 wherein said producing means includes afirst and second manometer having a given differential level and valvemeans for switching the liquid in said chamber means from one manometerto the other manometer to produce said step change.

6. An apparatus as in claim 5 wherein said given level is 10 centimetersand said step change is 10,000 microbars.

7. Apparatus as in claim 1 wherein said comparing means includes meansfor producing a signal which is a function of the ratio of said firstand second signals.

including means for shielding the 8. Apparatus as in claim 1 whereinsaid comparing means includes a first peak detector for receiving saidfirst signal, a first logarithmic amplifier for receiving the output ofsaid first peak detector and producing a third signal, a second peakdetector for receiving said second signal, a second logarithmicamplifier for receiving the output of said second peak detector andproducing a fourth signal, and a summing amplifier for receiving saidthird and fourth signals and producing a fifth signal which is afunction of the ratio of said first and second signals.

9. Apparatus as in claim 8 including a DC. meter for displaying saidfifth signal.

10. Apparatus as in claim 1 wherein said comparing means includes meansfor determining the sensitivity of said hydrophone.

11. An apparatus as in claim 1 including oscillator means connectedtosaid first tube for causing said tube to impart said sound to saidliquid.

12. An apparatus as in claim 11 including means to vary the frequency ofsaid oscillator.

13. A hydrophone testing apparatus comprising:

chamber means having a cylindrically shaped interior cavity forreceivingthe hydrophone to be tested and adapted to be filled with a liquid andhaving an inner cylindrical wall bounding said cavity,

a first cylindrical tube of piezo-ceramic material forming part of saidinner wall and extending about said cavity for imparting acousticalvibrations to said liquid,

a second tube of piezo-ceramic material forming part of said inner walland extending about said cavity axially aligned with said first tube forreceiving the sound imparted to said liquid by said first tube andproducing a first electrical signal,

variable oscillator means connected to said first tube for causingsaidfirst tube impart said vibrations to said liquid,

means for producing a second electrical signal from the vibrationsreceived from said liquid by said hydrophone,

means for calibrating said second tube by applying a step change inpressure to said liquid, and

means for producing a signal from said first and second signals which isa function of the ratio of said first and second signals.

14. A method of testing a hydrophone comprising the steps of:

disposing said hydrophone in chamber means having a cylindrical cavityfilled with a liquid bounded by an inner cylindrical wall with first andsecond axially aligned cylindrical tubes of piezo-ceramic material eachforming part of said wall and extending about said cavity,

electrically driving said first tube so that first tube impartsacoustical vibration to said liquid and comparing the electrical signalsproduced by said second tube with the electrical signals produced by'said hydrophone.

15. A method as in claim 14 including the steps of calibrating saidsecond hydrophone by applying a step change in pressure to said liquid.

16. A method as in claim 14 including the step of varying the frequencyof the signal driving said first tube so as to obtain the sensitivity ofsaid hydrophone over a given frequency range.

17. A method as in claim 14 wherein said step of comparing includes thestep of producing an electrical signal which is a function of the ratioof said signals produced by said hydrophone and said signals produced bysaid second tube.

18. A method as in claim 17 wherein said step of producing includes thesteps of applying the signals produced by said hydrophone to a firstpeak detector, applying the output of said first peak detector to afirst logarithmic amplifier, applying the signals produced by saidsecond tube to a second peak detector, applying the output of saidsecond peak detector to a second logarithmic amplifier, and applying theoutputs of said first and second logarithmic amplifiers to a summingamplifier ring.

* t I t

1. A hydrophone testing apparatus comprising: chamber means having acylindrically shaped interior cavity for receiving the hydrophone to betested, and adapted to be filled with a liquid, and having an innercylindrical wall bounding said cavity a first cylindrical tube ofpiezo-ceramic material forming part of said inner wall and extendingabout said cavity for imparting sound to said liquid, a secondcylindrical tube of piezo-ceramic material forming part of said innerwall and extending about said cavity axially aligned with said firsttube for receiving the sound imparted to said liquid by said first tube,and producing a first electrical signal, means for producing a secondelectrical signal from the sound received by said hydrophone beingtested and means for comparing said first and second electrical signalsto test said hydrophone.
 2. Apparatus as in claim 1 including means forshielding the ends of said tubes.
 3. An apparatus as in claim 1 whereinsaid liquid is water.
 4. An apparatus as in claim 1 including means forproducing a step change of pressure in said chamber means to calibratesaid second tube.
 5. An apparatus as in claim 4 wherein said producingmeans includes a first and second manometer having a given differentiallevel and valve means for switching the liquid in said chamber meansfrom one manometer to the other manometer to produce said step change.6. An apparatus as in claim 5 wherein said given level is 10 centimetersand said step change is 10,000 microbars.
 7. Apparatus as in claim 1wherein said comparing means includes means for producing a signal whichis a function of the ratio of said first and second signals. 8.Apparatus as in claim 1 wherein said comparing means includes a firstpeak detector for receiving said first signal, a first logarithmicamplifier for receiving the output of said first peak detector andproducing a third signal, a second peak detector for receiving saidsecond signal, a second logarithmic amplifier for receiving the outputof said second peak detector and producing a fourth signal, and asumming amplifier for receiving said third and fourth signals andproducing a fifth signal which is a function of the ratio of said firstand second signals.
 9. Apparatus as in claim 8 including a D.C. meterfor displaying said fifth signal.
 10. Apparatus as in claim 1 whereinsaid comparing means includes means for determining the sensitivity ofsaid hydrophone.
 11. An apparatus as in claim 1 including oscillatormeans connected to said first tube for causing said tube to impart saidsound to said liquid.
 12. An apparatus as in claim 11 including means tovary the frequency of said oscillator.
 13. A hydrophone testingapparatus comprising: chamber means having a cylindrically shapedinterior cavity for receiving the hydrophone to be tested and adapted tobe filled with a liquid and having an inner cylindrical wall boundingsaid cavity, a first cylindrical tube of piezo-ceramic material formingpart of said inner wall and extending about said cavity for impartingacoustical vibrations to said liquid, a second tube of piezo-ceramicmaterial forming part of said inner wall and extending about said cavityaxially aligned with said first tube for receiving the sound imparted tosaid liquid by said first tube and producing a first electrical signal,variable oscillator means connected to said first tube for causing saidfirst tube impart said vibrations to said liquid, means for producing asecond electrical signal from the vibrations received from said liquidby said hydrophone, means for calibrating said second tube by applying astep change in pressure to said liquid, and means for producing a signalfrom said first and Second signals which is a function of the ratio ofsaid first and second signals.
 14. A method of testing a hydrophonecomprising the steps of: disposing said hydrophone in chamber meanshaving a cylindrical cavity filled with a liquid bounded by an innercylindrical wall with first and second axially aligned cylindrical tubesof piezo-ceramic material each forming part of said wall and extendingabout said cavity, electrically driving said first tube so that firsttube imparts acoustical vibration to said liquid and comparing theelectrical signals produced by said second tube with the electricalsignals produced by said hydrophone.
 15. A method as in claim 14including the steps of calibrating said second hydrophone by applying astep change in pressure to said liquid.
 16. A method as in claim 14including the step of varying the frequency of the signal driving saidfirst tube so as to obtain the sensitivity of said hydrophone over agiven frequency range.
 17. A method as in claim 14 wherein said step ofcomparing includes the step of producing an electrical signal which is afunction of the ratio of said signals produced by said hydrophone andsaid signals produced by said second tube.
 18. A method as in claim 17wherein said step of producing includes the steps of applying thesignals produced by said hydrophone to a first peak detector, applyingthe output of said first peak detector to a first logarithmic amplifier,applying the signals produced by said second tube to a second peakdetector, applying the output of said second peak detector to a secondlogarithmic amplifier, and applying the outputs of said first and secondlogarithmic amplifiers to a summing amplifier to produce a signal whichis a function of the ratio of the signals from said hydrophone and thesignals from said second ring.